If custom alert sounds don't appear after upgrading to El Capitan

To continue using your custom alert sounds for Mail, Calendar, or other apps, you might need to put the sounds in a different folder.

If you upgraded to OS X El Capitan and an app no longer sees your custom alert sounds, put the sounds in the Sounds folder of your Library folder. In El Capitan, you can use either of these library folders:

The Library folder at the top level of your startup disk.

The Library folder in your home folder. To get to it, hold down the Option key and choose Go > Library from the menu bar.

If you don't have a Sounds folder in your Library folder, create one by choosing New Folder from the File menu. Spell "Sounds" with a capital S. You might be asked to enter your administrator password when creating the folder or moving sounds into it.

Restart your computer when done, then open your app and choose your custom alert sound again. If the app still can't see your sounds, make sure that the sounds are in the correct format for the app. For example, Calendar supports AIFF audio only.